THE ORANO DIKE SWARM OF ELBA ISLAND, ITALY: A LATE-PLUTONIC DIKE SWARM EMPLACED IN A TRANSFER ZONE

A magmatic center on Elba Island at the north end of the Tyrrhenian Sea lasted from 8.3 to 6.8 Ma, building a complex of nested Christmas-tree laccoliths, the 10 km-diameter Monte Capanne pluton with its late leucocratic dikes and pegmatites, and finally the Orano dike swarm (ODS). The swarm consists of strongly hybridized mantle-derived magmas constituting >90 km of dikes that intruded the pluton and its aureole, as well as several km of sedimentary rock above. The pluton is cut by a NE-trending geomorphic lineament (the Pomonte-Procchio line), SE of which late-plutonic aplite dikes filled NNE-trending fractures opened by the initial strain that later decapitated the complex with top-to-the-east décollement; aplite dikes NW of the line filled cooling fractures. The ODS intruded after aplite development but near the close of crystallization when at least part of the pluton was not yet wholly solid, and those dikes are restricted to the area NW of the PP line. ODS orientations are unrelated to either E-W stretching or pluton cooling, but instead are dominated by a major system trending N78E. These dikes are concentrated in belts through the pluton; between the belts, a minor system of Orano dikes dominates with N38W and N22E trends.

The regional geodynamic framework for the Elba magmatism consisted of an E-migrating compressive regime, with the subducted W-dipping Adria plate rolling back to trigger lithospheric delamination and development of an ensialic back-arc basin that also migrated eastward. Stretching segments marked by Neogene-Quaternary sedimentary basins are connected by NE-trending transfer faults that developed on inherited fractures previously active during the formation and destruction of the Tethys Sea. The Elba system lies on a segment of such a fault, and the ODS preserves the strain associated with deformation in that fault zone that concentrated magma flow to the build the Elba complex. Deformation along transfer fault segments in the Tyrrhenian region was short-lived as extension migrated NE. Magmatic centers in the Tuscan Magmatic Province are distributed along these lineaments and developed as a wave moving northeastward across the region.